I really think you're on to something.
We do not know factually he has cancer again (that IV is circumstantial at best) and the way he lies? Shiiiiiiiiiit.
It seems pretty sure that it's back. It was hinted in "Gliding Over All." Vince wouldn't have had hinted at it and then make it be a lie. The only iffy thing is that Walt said he had 6 months left to live. If my math correct, it has been 4 months since Walt's 51st birthday since Marie said to Skyler that it's been 3 months since they've had the kids, and the day they took the kids was on Walt's birthday. Then in tonight's episode Walt told Jesse he's been out of the game for a month now so that means the scene where Walt tells Skyler he's out and from when Hank finds out was a month apart.
So that's 4 months. But 6+4=/=12. Since he's still alive for his 52nd birthday that means he''s lived for more than 6 months, he's actually lived for 8 months.